In the operation of electrostatic precipitators it has been found that the precipitation efficiency is improved if high voltage pulses of short duration are superimposed on an underlying, relatively constant, base voltage. The reason for this improvement comes from the separation of functions of the electric field. The first function, provision of charge carriers for the charging of the particulates entrained in the gas, is performed by a corona discharge from at least one of the electrodes in the precipitator. This requires a very high electric field at the corona-emitting electrode. The field must be high, but it is not necessary that it be continuous. The second function, charging and transport of the particulates, requires a high average electric field which is most effective if continuously applied.
It is highly desirable to be able to vary these functions separately so as to maximize particulate entrapment for any given conditions of temperature, gas and particulate composition.
Pulse energization as a means of enhancing particle collection has long been recognized and several authors have published results of tests, c.f. Kumar, K. S., Feldman, P. L., Milde, H. I. and Schubert, C. J. "The results of first full scale utility demonstration of pulsed precipitation." 1979 Conference Record of IEEE-IAS-1979 Annual Meeting. The advantages of pulse energization are very significant because the corona is no longer solely dependant on the applied base voltage. It can, in addition, be controlled by pulse amplitude, pulse width and pulse frequency. This added dimension is especially important for highly resistive dust. To eliminate back corona, it is necessary to limit current density. Traditionally this is accomplished by reducing voltage. For these smaller allowable current densities, the corresponding voltage is very close to the corona onset point, producing poorly distributed corona along the discharge wires. There will thus be regions filled with excessive corona current and regions with little or none. It has been found that the application of infrequent pulses at levels well above the corona onset voltage eliminates the problem of nonuniform corona.
The application of pulsed energization technology to the collection of difficult dusts has been the subject of several engineering studies. However, its acceptance in industry has not been successful until recently because of a combination of problems related to reliability, economics and versatility of the available hardware.
The present invention seeks to improve the efficiency and lower the cost of the pulse generator by arrangement of the circuitry connecting the pulse generator to the precipitator and base voltage supply.